20250619 16:00 How 'C' Programs Work?

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How 'C' Programs Work? 

1. Write Code – Type your program in Turbo C.

2. Compile – Turn the code into machine language.

3. Run – See the output on the screen.

Popular Compilers

• Windows/MS-DOS: Turbo C, Turbo C++, Microsoft C

• Linux: gcc
  Turbo C/C++ is best for beginners.

Steps to Run in Turbo C

1. Open TC.EXE (usually in C:\TC\BIN)

2. File → New to start a new program

3. Type your code

4. Press F2 to save

5. Press Ctrl + F9 to run

6. Press Alt + F5 to see output

Turbo C creates a .EXE file that works on any PC!

Fix for Turbo C++ Error

If you see: “printf needs a prototype”, do this:

• Options → Compiler → C++ Options → Set CPP always

• Options → Environment → Editor → Set extension to .C

Taking Input with scanf()

Use scanf() to take values from the user:

int p, n;  
float r, si;  
scanf("%d %d %f", &p, &n, &r);  

• Use & before variable names (e.g., &p)

• Values must be separated by space, tab, or Enter key.

Sample Program

main() {
  int num;
  printf("Enter a number: ");
  scanf("%d", &num);
  printf("You entered: %d", num);
}

Types of C Instructions

1. Type Declaration – to define variables

2. Arithmetic – to do math

3. Control – to decide program flow (we’ll learn later)

Type Declaration in C

Before using a variable, you must declare its type:

int a;
float b;
char ch;

You can also declare and give values at the same time:

int x = 10, y = 20;
float a = 1.5, b = a + 2.0; // b must come after a!

But this won’t work:

int x = y = 10; //  y not defined yet

Arithmetic Instructions

C uses operators to do math:

+  -  *  /  %  (= assigns value)

Example:

int a = 10;
float b = 5.5, c;
c = b * 2 + 3;

• / = division

• % = remainder (only for int)

Types of Math Statements

1. Integer – all numbers are integers
   int x = 10 + 20;

2. Float (Real) – all numbers are decimal
   float x = 3.2 / 1.6;

3. Mixed – both int and float
   float avg = (a + b) / 2;

Important Tips

• Only one variable allowed on left of =

• % doesn’t work with float

• 'A' stores ASCII value (like 65)

• You can do math with char, int, float

• No ^ or ** for power! Use:

#include <math.h>
int x = pow(3, 2); // 3^2 = 9

Integer & Float Conversion in C

When doing math in C:

• int + int → result is int

• float + float → result is float

• int + float → result is float (int is converted to float first)

Examples:

5 / 2       → 2      (int result)
5.0 / 2     → 2.5    (float result)
5 / 2.0     → 2.5    (float result)

Conversion in Assignment

Sometimes types don’t match on both sides of =. So C converts them:

int i;
float b;

i = 3.5;   // 3.5 becomes 3 (float → int)
b = 30;    // 30 becomes 30.0 (int → float)

Even with big formulas, same rules apply!

Important:

• int cuts off decimal part (no rounding!)

• float can store decimals

• Mixed math = float result

• Assigning float to int = decimal is dropped

Example:

int k;
float a;

k = 2 / 9;     // = 0
a = 2 / 9;     // = 0.0

k = 2.0 / 9;   // = 0.222... → becomes 0
a = 2.0 / 9;   // = 0.222...